This invention relates generally to substrate bonding techniques. More particularly, the invention relates to techniques for providing high strength and high quality bonding techniques for fabricating multilayered substrates. Such substrates include semiconductors, silicon-on-insulator substrates, compound semiconductors, ceramic packaging materials, and metallic and nonmetallic films. The substrates can be used for the manufacture of semiconductor devices, MEMS, photonic devices, waveguides, ink head and other dispensing devices, polymeric coated or polymeric devices, and laminated devices, as well as biomedical applications.
Conventional wafer bonding includes xe2x80x9cdirectxe2x80x9d wafer bonding of silicon wafers, which attach themselves to each other by placing faces of such wafers directly together. Direct bonding has often been considered as an alternative to using organic or inorganic bonding agents for bonding silicon and a number of other semiconductor materials. For example, direct bonding can be facilitated by first activating the surface of the wafer with a NH4OH based solution for silicon and its oxides. For nitrides such as AlN and Si3N4 an acid bath or an HF dip might be used. Plasma exposure has been another technique for activating the surfaces of wafers to be bonded. A pioneering technique that has been developed for such plasma exposure is described in U.S. Pat. No. 6,180,496, entitled xe2x80x9cIn Situ Plasma Wafer Bonding Method,xe2x80x9d in the name of Farrens, et al., commonly assigned, and hereby incorporated by reference for all purposes. Such surface activation methods render the wafer surfaces hydrophilic and amenable to bonding. After surface activation, the wafers can be placed in a spinner where they may be rinsed in de-ionized water or other chemical agent. The wafers are then placed surface to surface, at which point van der Waals forces pull the two wafers into contact.
The contact bonds, which are formed in accordance with conventional wet surface activation, are generally weak, and not suitable for device processing. This is because the process of oxidation (or corrosion of any kind), which is the underlying mechanism of all high temperature direct bonding of semiconductor materials, is the result of a two step process: (1) migration of the reacting species to the reaction site and (2) the subsequent chemical reaction itself. The energy that must be supplied to the xe2x80x9csystemxe2x80x9d to cause the silicon and the oxygen atoms to migrate and react is quite large, and as such this particular reaction is not self-sustaining at low temperatures. Therefore, the bonds have been typically strengthened by high temperature anneals for silicon and its oxides, and moderate temperature anneals for nitrides.
Other limitations also exist using conventional bonding techniques. For example, plasma activation bonding of substrates can cause plasma damage to surfaces of the substrates before bonding. Such damage can lead to lower bond quality as well as resulting damage to devices fabricated on such bonded substrates. Some of the damage caused by plasma activation can be repaired, at least in part, by heat treatment of the bonded substrates. Heat treatment, however, is not generally desirable for the manufacture of substrates using conventional layer transfer techniques using hydrogen species. Such hydrogen species often diffuse out of the substrate during high temperature thermal treatment, which leads to difficulty in transferring such layer using one of a variety of separation techniques. These and other limitations of conventional bonding techniques have been described throughout the present specification and more particularly below.
From the above, it is seen that an improved technique for bonding substrates together is highly desirable.
According to the present invention, techniques for bonding substrates together are provided. More particularly, the invention provides a method and apparatus for using multiple frequencies in a plasma environment for activating surfaces of substrates before bonding them together. Although the invention has been applied to the manufacture of semiconductor wafers, it would be recognized that the invention has a much broader range of applicability such as the manufacture of semiconductor devices, MEMS, photonic devices, ink head and other dispensing devices, polymeric coated or polymeric devices, waveguides and other photonic applications, and laminated devices, as well as biomedical applications.
In a specific embodiment, the invention provides a method for bonding surfaces of different substrates together. The method includes providing a first substrate having a first surface to be bonded. The method also includes supplying a gas; and igniting the gas to produce plasma using a first frequency signal (e.g., RF frequency) from an electro-magnetic source. The method also includes producing a second signal and applying the second frequency signal (e.g., RF frequency) to the plasma to maintain the plasma on the first substrate. The first surface of the first substrate is exposed to the plasma being driven by the second frequency without substantial etching of the first surface. In some embodiments, this process may be repeated for the first surface of the second substrate although this is not needed for all applications. The method then contacts the first surface of the first substrate to a first surface of a second substrate to produce a bond there between.
In an alternative embodiment, the invention includes an apparatus for bonding substrates together. The apparatus has a work chamber for receiving therein one or more substrates. The apparatus also has a first electrically conductive platen disposed in the work chamber; and a second electrically conductive platen disposed in the work chamber and spaced apart from the first platen by a distance d. A first signal feedthrough is coupled to the first platen for receiving a first frequency signal (e.g., RF). A second frequency signal (e.g., RF) feedthrough is coupled to the second platen for receiving a second frequency signal having a frequency less than a frequency of the first frequency signal. The work chamber has a gas inlet for receiving a gas. The gas is ignited to form a plasma by the first frequency signal. A surface of a substrate is disposed in electrical contact with the second platen is exposed to the plasma. The first signal is also such that the plasma will not substantially etch the surface, since it is maintained away from the surface. In some embodiments, two or more wafers are in the chamber and such wafers are biased to the second platen at the second frequency, where they are simultaneously activated. After such activation, they can be joined together in the chamber or removed to a separate chamber for joining, which bond the wafers together.
Numerous benefits are achieved by way of the present invention. For example, the invention can be used with conventional process technology. Additionally, the invention can be used to effectively bond wafers together without substantially damaging them using plasma tools. Here, the damage is the type that causes undesirable yield and/or quality problems (e.g., unacceptable). In other embodiments, the invention can produce a high quality bond at lower temperatures, which is effective for certain layer transfer operations, such as a controlled cleaving processes or a thermal separation process. Depending upon the embodiment, there can be one or more of these benefits. These and other benefits are described in more detail throughout the present specification and more particularly below.
The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings: